Abstract

Phase equilibria were investigated in the pressure range 400–2000 MPa on the Pinatubo dacite, a representative example of natural Cenozoic adakites (slab melts), to constrain the temperature versus wt% H2O conditions of magma genesis. The experimental results suggest that water contents of the melt were at least 10 wt% and temperatures were ∼900 °C during magma ascent from the slab. Natural aluminous amphiboles present in the dacite crystallized at pressures of ∼1000 MPa, fixing an upper temperature limit of 950 °C for the slab melts at the crust-mantle boundary. Amphiboles experimentally crystallized at 1000 MPa are identical to those found in melt inclusions in metasomatized mantle xenoliths. These natural inclusions represent samples of slab melts and contain glasses with a dacitic bulk composition, which requires an H2O content in the melt of >10 wt% at 1000 MPa and 900 °C. For conditions of magma generation of 800–900 °C at 2000–2300 MPa, melt H2O contents close to 15 wt% are needed to generate melts with dacitic bulk composition. Overall, these constraints are inconsistent with slab melts being produced by dehydration melting of amphibolite, a mechanism unable to produce dacite melt compositions that are both hydrous and cool. Slab melting in modern subduction zones must occur under fluid-present conditions at temperatures below 900 °C, in agreement with the thermal regime of subducted oceanic crust as deduced from numerical simulation of heat transfer using the present-day mantle geotherm. At such low temperatures, Ti-bearing phases are stable during melting; thus any metasomatizing slab melt will be strongly depleted in Nb, Ta, and Ti, a major characteristic of arc magmas produced in the mantle wedge.